XML : Android Navigation Toolbar Profile Fragment

I'm very new to Android Development, and I've been playing around with the default Navigation Toolbar template that Android Studio provides. I've followed tutorials online with regards to how to set up fragments for the elements of the menu. So for the camera, gallery, etc. I'm able to switch to the fragment I want. However, I also want to make the green profile picture area also switch to the proFragment (profile fragment) when it's clicked (this is under the nav_header_main.xml file), but I can't seem to figure out how to. I have an id set for that section, and I try to reference it like the template did when it referenced the floating action button:

          LinearLayout user= (FloatingActionButton) findViewById(R.id.user_layout);      fab.setOnClickListener(new View.OnClickListener() {          @Override          public void onClick(View view) {              proFragment fragment = new proFragment();              android.support.v4.app.FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();              fragmentTransaction.replace(R.id.fragment_container, fragment);              fragmentTransaction.commit();        });    

However, my app crashes and spits out the error:

  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.testApp/com.example.android.testApp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.setOnClickListener(android.view.View$OnClickListener)' on a null object reference    

I've also tried putting an invisible button over the area, but I keep getting " java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.testApp/com.example.android.testApp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference".

I'm confused to why I'm referencing a null object - is it because the green user profile header hasn't been created yet? Thanks for all the help!!